There has hitherto been proposed an apparatus that measures the number of microorganisms existing in a specimen by staining the specimen with a fluorescent staining reagent and irradiating the specimen with excitation light to emit fluorescence (see, for example, Japanese Unexamined Patent Application Publication No. 2008-145276 (PTL 1)). In the apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2008-145276 (PTL 1), the excitation light applied to the specimen and the fluorescence emitted from the specimen are coaxial with each other, the fluorescence is caused to arrive via a filter at an optical system for receiving the fluorescence, and the excitation light is removed by the filter.